"There was an abrupt deceleration, the lights went off then there was a spark of fire and smoke, we were blocked in," said one passenger.

"I was in one of the carriages, I felt a sudden shock and fell down, all the people were lying side by side on the floor. I cannot speak for the others, I was in the front carriage, the carriages were not very full but there were enough people there."

Russia's investigative committee said it was looking into the causes of the accident.

The committee, which answers to Mr Putin, said it had opened a criminal case on suspicion of failure to meet safety guidelines but that it had not yet determined the cause of the accident

Critics have accused the Moscow government of spending too much on extending the underground network and not enough on maintenance.

"There is no-one alive left [underground]," Moscow's deputy mayor Peter Biryukov said. "The cause is not known, the work continues."

Bodies remain underground as investigators search for answers

Mr Biryukov said bodies were recovered from the wreckage, but that some remained underground.

Television footage showed rescue workers carrying the injured away on stretchers, while paramedics treated some on nearby grass verges.

"It braked very hard. The lights went off and there was lots of smoke," a man at the scene told Rossiya-24 television.

"We were trapped and only got out by some miracle. I thought it was the end.

"Many people were hurt, mostly in the first rail car because the cars ran into each other."

A city transport services spokesman told news agency Interfax that all passengers had been evacuated from the affected stations, dismissing reports that some passengers were still trapped in the tunnel.

Moscow mayor Sergei Sobyanin was at the scene and pledged to do everything possible to help the victims.

Russians regularly criticise the country's transport safety record. Recent disasters included the 2011 sinking of a ferry boat that killed 128 and an airplane crash that killed the Lokomotiv Yaroslavl ice hockey team the same year, both of which were blamed on lax safety regulations and technical errors.

On weekdays as many as 9 million people use Moscow's subway system.

Famed for its high-vaulted halls adorned with Soviet socialist realist art, the underground network has expanded from 13 stations opened in 1935 to 194 stations across the megalopolis today.

Islamist militants have previously carried out deadly attacks in Moscow, including twin suicide bombings that killed 40 people on the subway in 2010.